1. Field of the Invention
The present invention relates to display units for use in computer systems, and more specifically to a method and apparatus for displaying images at a refresh rate less than the rate at which the images are encoded in a received display signal.
2. Related Art
Display units are often used in computer systems for displaying images. A typical display unit receives a display signal comprising display data and synchronization signals. The display data contains image frames and the synchronization signals indicate the separation of the image frames and the lines within each image frame. A display unit displays the encoded images.
Image frames are often encoded at a rate, which may be referred to as encoding rate. That is, encoding rate specifies the number of image frames received in a given duration, for example a second. As an illustration, under the PC-98 specification by Microsoft Corporation, analog display signals may contain image frames encoded at 75 Hz encoding rate.
Display units often contain a display screen, and the display screen is refreshed with images encoded in a received display signal. The rate at which the images are refreshed may be referred to as a refresh rate. A target refresh rate is generally associated with each display unit. The target refresh rate is usually specified by the manufacturer, and determined by the implementation of the display screen and associated interfaces. The target refresh rate is often limited to minimize the overall cost of implementation of a display unit. For example, flat monitors are often implemented for 60 Hz target refresh rate.
The target refresh rate is some times less than the encoding rate. Under such circumstances, a display unit may need to convert the image frame rate from the encoding rate to the target refresh rate, and the process may be referred to as frame rate conversion. Display units typically employ frame buffers for frame rate conversion.
Typical frame rate conversions entail generating pixel data elements representing image frames encoded in a received display signal, and retrieving the pixel data elements at a different frame rate. In this context, the image frames encoded in a display signal may be referred to as source image frames and the image frames displayed may be referred to as displayed image frames.
In a known prior scheme, a television display unit may employ a frame buffer having sufficient memory capacity to store data representing a single source image frame and convert the frame rate by a factor of 1/Z (wherein Z is an integer). Typically, only one of every Z source image frames is displayed and the remaining (Z-1) source image frames may be ignored.
However, such an approach may not be suited in digital display units which typically require frame rate conversions by a factor not equal to an integer. In the examples noted above, the frame rate may need to be converted from 75 Hz (PC-98 Standard) to around 60 Hz. In addition, it may be a requirement that the same display unit operate with different encoding rates (e.g., 75 Hz, 72 Hz, and 90 Hz), which also may not be practicable with this approach.
An alternative embodiment may employ a frame buffer with memory space to store one image frame, and attempt to retrieve pixel data elements at a desired refresh frame rate. However, using such limited amount of memory space may lead to image tearing. In general, image tearing refers to display artifacts which may be generated if one portion of a displayed image is generated from one source image frame, and the other portion is generated from another source image frame. A displayed image may be generated from two source images as the data corresponding to a subsequent encoded image frame replaces the data corresponding an earlier source image frame before the displayed image (or image to be displayed) is completely generated.
At least to overcome the image tearing problem, a display unit may employ a larger frame buffer for frame rate conversion. For example, a frame buffer having sufficient storage for two source image frames may be used. The display unit may ensure that a source image frame is not partially retrieved for use in a display by using the other stored frame. Accordingly, the image tear problem may be avoided.
However, one problem with such a solution is it may not be feasible to cost-effectively integrate such large frame buffers into a single integrated circuit along with other components generating display signals for a display screen. Integration may be important in digital display units, for example, to minimize the manufacturing costs and the amount of space used.
Accordingly, what is needed is a flexible approach which enables a display unit to display images at a refresh rate which is lower than the encoding rate used in a received display signal. The images may need to be displayed without artifacts such as image tears while not requiring substantial memory space in frame buffers. In addition, the display unit may need to operate with several encoding rates.